Strange axis feeding issue

[TeachMePlease]

If that's 18 TPI and I see 11 threads, then you've got approximately .611" of thread. Horn makes 60° threading inserts that will go in a 3/4" shank round holder that should go in your back working tooling easily (they also make Star specific holders that replace the holders you've got, so you can shorten your stickout if necessary). The inserts go up to 25mm reach. 20mm should do you just fine. I'd suggest looking at Horn part number L105.0205.3.4 for a left handed threading bar and the appropriate holder for it (3/4" shank would be BU105.0750.01). This would get you the correct insert geometry and eliminate that as a possibility.

 

[wmpy]

What is the material? Have you tried running a few parts in softer material (aluminum) to see if it still happens? I'd be just as inclined to wonder if the bars have hard spots or poor distribution of alloying materials that are causing the material to walk around in X as you're threading.

I tried machining the thread with a piece of brass. It made a really pretty looking thread, but it still had the same problem with the pitch diameter being up and down over the length. I only made one, but it did seem like the variation in pitch diameter was better than with the 416, but it was still there.

 

[wmpy]

I'm no Fanuc expert by any means, but I think the 2 encoders "compare notes" and will throw an alarm if they're on different pages. How's the other pulley and belt look?

I also don't know if removing the encoder will disturb anything? it looks like it can be clocked?

The belt looks great. I can't see the other pulley, but sticking my fingers up there and feeling around, it checks out.

 

[wmpy]

Any chance your sub collet isn’t tight enough and the part is slipping?

Sub-collet is plenty tight.

 

[wmpy]

Those numbers look fine to me.
Do you have a dummy piece where you can run it about 1000 rpm to see if there is a change?

I ran one at 1000 rpm, and it behaved exactly the same except for sounding quite awful for a few of the middle passes.

 

[wmpy]

I was gunna say to check collet pressure as well, but then I went back and looked at the pic for a while aggin, and - nah - not collet.

By the same regards, I'm gunna rule out thrusts as well.

I would say that your ball nut could possibly be a thing?
Maybe you have a nut 1/2 full of bad balls, but I would think that you'd see other issues as well.

I'd start by slowing it down.
And then I start swapping encoders.

But since this thread is a few days old, I'm guessing that you've done something by now?


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I am Ox and I approve this here post!

In looking into this, I did just find that I have about .0025" play in my Z. I wasn't aware of this and was quite surprised by how much there was. It hadn't shown up in any other work I've been doing with the sub. Like I said in my first post, I'm mostly just picking off, spotting, drilling, chamfering, and rotary broaching with the sub. None of that type of work is very critical in the Z.
I was in a rush when I was checking, so I need to get back in there to see where the slop is actually coming from. There was very little backlash when I measured that, but I suppose it could have been comped out. I measured the slop by just pushing and pulling on the sub.
I wouldn't immediately expect Z play to cause the threading problem I'm seeing. Plenty of sloppy lathes are able to make good threads. But if it's in the ball screw like was suggested, this could be the problem.
At this point I'm not sure whether to pursue the mechanical or electrical solution.

 

[wmpy]

Both. The velocity and position of the axis are critical for a proper thread. The spindle performance also is critical. Typically there is an encoder driven by the spindle that provides an index pulse (one revolution pulse) for starting each thread pass as well as a pulse train that the spindle drive uses to control the spindle speed.

On a machine like this it's easy to view encoder signals with an oscilloscope. Personally, that's what I would do to begin troubleshooting the control system. I'd only do that after confirming that there were no tooling or mechanical issues though. Belt driven dual encoder feed back systems can be troublesome if there is lost motion (loose or bad belt, worn pulleys) in the belt or pulleys. If the velocity loop and position loop are not pretty closely in sync, the control and drive can get into an oscillation.

Thanks for the info. Unfortunately, checking encoders with a scope is out of my realm of knowledge. I may need to find someone who knows what they're doing.

 

[wmpy]

If that's 18 TPI and I see 11 threads, then you've got approximately .611" of thread. Horn makes 60° threading inserts that will go in a 3/4" shank round holder that should go in your back working tooling easily (they also make Star specific holders that replace the holders you've got, so you can shorten your stickout if necessary). The inserts go up to 25mm reach. 20mm should do you just fine. I'd suggest looking at Horn part number L105.0205.3.4 for a left handed threading bar and the appropriate holder for it (3/4" shank would be BU105.0750.01). This would get you the correct insert geometry and eliminate that as a possibility.

Thanks for the suggestion, Teach. I was going to look into the proper threading setup for the sub, but you already did the legwork for me. I appreciate it. I am going to see if they offer this in a 22 mm shank because that's what Star decided would be the best shank size to standardize on.

 

[Ox]

22?

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I am Ox and I approve this post!

 

[TeachMePlease]

Thanks for the suggestion, Teach. I was going to look into the proper threading setup for the sub, but you already did the legwork for me. I appreciate it. I am going to see if they offer this in a 22 mm shank because that's what Star decided would be the best shank size to standardize on.

Horn makes Star Specific holders that take the place of the blocks that hold your round shank sub and back working tools. Go to this page and scroll down almost all of the way until you find the catalog for "Catalog back working tools for Star automatic lathes"